ABSTRACTTransient or intermittent releases of specific volatile or semi-volatile organic compounds are not managed well by existing predictive fate models such as TOXCHEM+ or Water9 because they are based on steady-state conditions. In this paper, two scenarios for treatment of transient organic compound concentrations are studied using dynamic simulation:

Remedial actions (such as nutrient addition biodegradation, and use of off-line storage with slow return pumping to treatment) are compared to passive acceptance of the release in treatment.

For covered processes, strategies for managing headspace concentrations which may approach lower explosive limits are examined. Certain remedial strategies prove more effective in minimizing air emissions and effluent releases than others, demonstrating the benefits of the dynamic modeling approach.

INTRODUCTIONWastewater treatment plants are subjected to fluctuating concentrations of trace organic and metal contaminants (Monteith, 1986). In industrial settings, batch operations in different processing areas are common, and contribute short-term waste loadings of variable composition to the industry’s wastewater treatment facilities. The mix of industrial solvents, process materials and reaction by-products from different production schedules can result in a highly variable mix of contaminant concentrations. While such variations are “normal” operation, industrial wastewater treatment facilities may also be subject to treatment of accidental releases of product or raw material to mechanical failure, disaster or human error.

General fate models (GFMs) such as TOXCHEM+ (Hydromantis, Inc.) and WATER9 (U.S. EPA) are used to predict the fate of specific organic and metal contaminants in wastewater treatment. To date, these GFMs are constructed as steady-state models. As such, they represent an average condition, rather than a more accurate and realistic assessment incorporating timedependent dynamic conditions. Steady-state modeling is accepted by regulatory agencies for reporting emission rates on a monthly or annual basis, as this application represents the “average” conditions for the time period.

There are other applications, however, that require an assessment beyond the capability of steady-state models. Incidents of a short-term variable nature, such as an accidental release of materials used in a process (e.g. valve left open, ruptured gasket or a crack in heat exchanger elements) can affect the operation of a treatment system or the health and safety of personnel employed in the wastewater treatment area. At such times, a dynamic process simulator may provide a much more useful to determine the appropriate response or strategy for dealing with the transient release.

This paper describes the benefits of combining the TOXCHEM+ predictive fate software with the dynamic general purpose simulator (GPS-X) for investigating and responding to a transient organic contaminant concentrations in wastewater treatment, based on an actual situations encountered at petrochemical plants.

OBJECTIVEThe goals of this paper are to demonstrate the importance of coupling of a dynamic wastewater treatment simulator with models for (a) predicting the fate of volatile organic compounds or hazardous air pollutants on a time-varying basis during wastewater treatment; and (b) providing guidance for optimal management of transient organic compound levels in wastewater treatment.

BACKGROUNDWastewater Treatment Modeling and SimulationNumerical modeling of the activated sludge process is a common engineering analysis tool that has gained popularity and acceptance over the past two decades. There are two commonly-used, and essentially different, approaches to modeling treatment systems in use today: steady-state and dynamic. Steady-state simulation is a more straight-forward approach that assumes all inputs to the system are constant, and solves for a result that is representative of the system at equilibrium. Dynamic models allow for time-varying input, and can simulate how a system responds to changes in driving forces, internal operation or other external stimuli.

TOXCHEM+ predictive fate software employs the steady-state approach. It uses mass balance and kinetic rate expressions to estimate the behaviour of specific trace organic contaminants in wastewater treatment. The mathematical models in TOXCHEM+ are written in a steady-state format, and therefore are not suitable for evaluating dynamic conditions in wastewater treatment systems, as typified by a temporary spill condition. The basis for the modeling equations in TOXCHEM+ have been discussed by Melcer, et al. (1994).

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